OPA861
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SBOS338–AUGUST 2005
ACTIVE FILTERS USING THE OPA861 IN
CURRENT CONVEYOR STRUCTURE
output becomes the input, which is excitated by a
current source. The following equation describes the
interreciprocal features of the circuit: VOUT/VIN
=
One further example of the versatility of the Diamond
Transistor and Buffer is the construction of
high-frequency (> 10MHz) active filters. Here, the
Current Conveyor structure, shown in Figure 40, is
used with the Diamond Transistor as a Current
Conveyor.
IOUT/IIN. Resistances and capacitances remain un-
changed. In the final step, the operational amplifier
with infinite input impedance and 0Ω output im-
pedance is transformed into a current amplifier with
0Ω input impedance and infinite output impedance. A
Diamond Transistor with the base at ground comes
quite close to an ideal current amplifier. The
well-known Sallen-Key low-pass filter with positive
feedback, is an example of conversion into Cur-
rent-Conveyor structure, see Figure 43. The positive
gain of the operational amplifier becomes a negative
second type of Current Conveyor (CCII), as shown in
Figure 40. Both arrangements have identical transfer
functions and the same level of sensitivity to devi-
ations. The most recent implementation of active
filters in a Current-Conveyor structure produced a
second-order Bi-Quad filter. The value of the resist-
ance in the emitter of the Diamond Transistor controls
the filter characteristic. For more information, refer to
application note SBOS047, New Ultra High-Speed
Circuit Techniques with Analog ICs.
The method of converting RC circuit loops with
operational amplifiers in Current Conveyor structures
is based upon the adjoint network concept. A network
is reversible or reciprocal when the transfer function
does not change even when the input and output
have been exchanged. Most networks, of course, are
nonreciprocal. The networks of Figure 41, perform
interreciprocally when the input and output are
exchanged, while the original network, N, is
exchanged for a new network NA. In this case, the
transfer function remains the same, and NA is the
adjoing network. It is easy to construct an adjoint
network for any given circuit, and these networks are
the base for circuits in Current-Conveyor structure.
Individual elements can be interchanged according to
the list in Figure 42. Voltage sources at the input
become short circuits, and the current flowing there
becomes the output variable. In contrast, the voltage
IOUT
VOUT
E
B
C
−
CCII
+1
C
C
VIN
R
R
R
R
IIN
C/2
C/2
4KQ2/R2C2
VOUT IOUT
=
T(s) =
=
2
2
2
−
VIN
IIN
s2 + 2/RC[2Q(1 K) + 1]s + 4KQ /R C
Figure 40. Current Conveyor
19
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